Heretofore, ammonium polyphosphate has been used and added to paper, wood, synthetic resins or the like as a component for a flame retardant used therein. However, ammonium polyphosphate is water-soluble by nature and therefore, has such characteristic properties that it is highly susceptible to hydrolysis under a high temperature condition. A resin composition in which such un-treated ammonium polyphosphate is blended or an article obtained by molding the resin composition is also inferior in water resistance due to the water-solubility of ammonium polyphosphate and is limited in the practical applications. Moreover, the un-treated ammonium polyphosphate is insufficient in the affinity for synthetic resins and accordingly, the molded article obtained using a resin composition to which the ammonium polyphosphate is added as a component for a flame retardant suffers from a problem in that the mechanical strength, appearance or the like of the molded article are largely impaired due to the presence of the polyphosphate.
To eliminate the drawbacks peculiar to the ammonium polyphosphate, there have been some proposals. Ammonium polyphosphate which is subjected to coating or encapsulation with a thermosetting resin is disclosed in, for instance, U.S. Pat. Nos. 4,467,056, 4,347,334 and 4,514,328; Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") Nos. Hei 4-20944 and Hei 4-55625; and Japanese Un-Examined Patent Publication (hereinafter referred to as "J.P. KOKAI") Nos. Sho 61-98719, Sho 61-98721, Sho 61-98722, Sho 61-101514, Hei 6-24719 and Hei 7-277713. In addition, ammonium polyphosphate which is treated, coated or modified with a melamine monomer is disclosed in J.P. KOKOKU Nos. Sho 52-39930, Sho 53-15478 and Hei 5-50536; U.S. Pat. No. 4,515,632; and Registered Japanese Patent No. 2,598,742. J.P. KOKAI No. Hei 3-131508 and PCT No. 9,208,758 disclose ammonium polyphosphate coated with a silicone resin. U.S. Pat. Nos. 5,071,901, 5,109,037, 5,162,418 and 5,164,437 disclose ammonium polyphosphate which is treated with a surface-treating agent.
However, all of the ammonium polyphosphate products produced according to the processes disclosed in the aforementioned prior arts suffer from problems in that they are insufficient in the affinity for thermoplastic resins and dispersibility therein and that the articles obtained by molding the compositions prepared by blending the products into resins have considerably reduced mechanical strength. In order for eliminating these problems, a halogen-free flame retardant which is coated with or encapsulated in a thermoplastic resin has recently been proposed in J.P. KOKAI Nos. Hei 9-40876 and Hei 9-13037.
However, the halogen-free flame retardant which is coated with a thermoplastic resin according to the process disclosed in these patents suffers from problems in that it is still insufficient in the chemical resistance such as acids, alkalis and organic solvents and that if the core material comprises ammonium polyphosphate, the flame retardant is not improved even in its water resistance.
The inventors of this invention have conducted various studies to obtain ammonium polyphosphate which is excellent in water resistance, resistance to acids, resistance to alkalis, resistance to ions and resistance to organic solvents and which can provide an article whose mechanical strength is not impaired to a considerable extent, when a composition is prepared by blending ammonium polyphosphate into a thermoplastic resin as a component of a flame retardant used therein and then molded into the article. As a result, the inventors have found out that the foregoing problems can be solved by providing a substance which comprises a specific core material containing such ammonium polyphosphate and a thermoplastic resin coating the core material, in particular, a coating layer of a thermoplastic resin consisting of a cross-linked polymer of a specific monomer having, in the molecule, 1 to 5 groups each carrying polymerizable double bond and have completed the present invention on the basis of the foregoing finding.
As will be clear from the foregoing, an object of the present invention is to provide a substance mainly comprising ammonium polyphosphate, which is excellent in water resistance and chemical resistance, which can provide an resin molded article whose mechanical strength is not impaired to a considerable extent by blending ammonium polyphosphate into a thermosetting or thermoplastic resin as a component of a flame retardant used therein and which has a high hygroscopicity-controlling effect, as well as a process for producing the same and a flame retardant containing the same.